A conventional motor vehicle transmission with a manual, stepped ratio gearbox typically has a user operable clutch between the engine and the gearbox serving to couple/decouple them. When the vehicle is launched—that is, when it moves away from rest—there is an inevitable initial mismatch of speeds between the transmission input and the engine output, and by allowing the clutch to slip at this stage the driver causes torque to be applied to the transmission to accelerate the vehicle, without applying an unsustainable load to the engine that would cause it to stall. In the case of conventional automatic, stepped ratio transmissions the initial mismatch of speeds is accommodated by a torque converter. The clutch and torque converter are examples of what will be referred to herein as “launch devices”.
In some continuously variable transmissions (“CVTs”) no launch device is required. Instead, such CVTs are able to provide an infinite speed reduction. That is to say, by adjustment of ratio and without physically decoupling the engine from the transmission output, they can achieve a condition in which the transmission output is stationary while the engine is running. This condition is referred to as “geared neutral”. Launch can be achieved simply by moving the CVT ratio away from geared neutral, and it is not necessary to decouple the transmission from the engine to bring the vehicle to a halt.
The inventors have recognised, however, that it would be advantageous if such a transmission could be made to emulate in some respects the functionality of a manual transmission having a user operable clutch. The use of a clutch is very familiar to many drivers. It offers positive assurance that no creep torque will be applied to the vehicle wheels. It is also useful e.g. in certain low speed manoeuvres.
Note that the word “engine” is used herein for the sake of brevity to refer to any form of rotary driver, and must be understood to encompass not only internal combustion engines but also electric motors, external combustion engines etc.
The word “variator” will be used herein to refer to a device that has a rotary input and a rotary output and that transfers drive from one to the other at drive ratio that can be continuously varied. Such a device is to be found in any CVT. Most, if not all, variators have some movable torque transfer part which is involved in the transfer of drive and whose position corresponds to the variator ratio. In the well known case of a toroidal race, rolling traction type variator, rollers serve as the movable torque transfer parts. They transmit drive from one toroidally recessed race to another, and their motion involves a change in roller inclination which is associated with a change in variator drive ratio. A force is applied to the movable torque transfer part to influence its position, and so to influence variator drive ratio. In principle, this force could be provided through some direct linkage to a user operable control such as a lever or foot pedal. In practice, excepting the case of very lightweight vehicles, the force required—which is related to the torques suffered by the variator—proves to be too large to be comfortably provided by the user. Achieving geared neutral with such an arrangement could also be problematic, since any minor deviation of the position of the torque transfer part from the position required for geared neutral would cause the transmission to adopt a very low drive ratio, potentially resulting in a large “creep torque”, albeit at low wheel speed.
Modern CVTs are typically reliant upon sophisticated electronic controllers to regulate the engine and the transmission. However there is a commercial need for simple physical devices for controlling CVTs. These would be particularly attractive for use in basic model tractors, for example.